Anchor

ABSTRACT

A marine anchor including a main fluke with a shank attached to the fluke. To enable the anchor to orientate on the sea bed from an inverted position to an upright working position, auxiliary flukes are supported by respective lugs upstanding from the fluke on either side of the anchor&#39;s vertical plane of symmetry. The auxiliary flukes extend laterally from the lugs and are inclined at an acute angle to the main fluke. The anchor includes support means arranged such that when the anchor lies inverted on a firm seabed one of the auxiliary flukes is located in position for digging in when the anchor is pulled forwardly.

DESCRIPTION

The present invention relates to an anchor for mooring a floating objectto a mooring bed.

A fixed fluke anchor having a cranked shank which is capable oforientating to an upright digging attitude automatically by rolling whendragged inverted over most mooring bed surfaces is known from UK PatentsNos. 1356259 and 1513453. However, in certain mooring beds of firm clay,the rolling action can be impeded by the flat sides of the shank bearingagainst the mooring bed surface when the anchor has rolled approximately45 degrees onto its side.

It is an object of the present invention to obviate or mitigate thisproblem.

Marine anchors as described above have a fluke angle θ defined by theangle between a line extending from the anchor cable attachment point onthe shank to the rear of the fluke and a fluke central line. The flukecentral line is in fact constituted by the line intercept of a verticalsymmetry plane of the anchor with the upper surface of the fluke, such aline intercept defining a forward direction. If the upper fluke surfaceis flat then the line intercept is a straight line: however if the flukeupper surface is longitudinally curved giving a curved intercept thenthe pertinent straight fluke intercept line would be the chord subtendedby the "curved" intercept. In this specification, this chord isidentified by the expression "fluke central line". Also, similar flukelongitudinal intercept lines could be obtained on the fluke by planesparallel to said vertical symmetry plane. The above defines theexpression "central fluke line or longitudinal fluke line ashereinbefore defined" used in this specification. The fluke angle θ canbe within the range 25 degrees to 55 degrees.

According to the present invention there is provided an anchorcomprising a fluke member arranged transversely to a longitudinal planeof symmetry of the anchor, a shank adapted at one end for attachment toan anchor line and at the other end for attachment to said fluke member,said fluke member including an upstanding lug on each side of said planeof symmetry; auxiliary flukes each attached to a respective lug so as tobe remote from the plane of symmetry, each auxiliary fluke extendingsubstantially transversely to said plane of symmetry and providing aleading surface inclined upwardly at an acute forwardly opening angle β,to a plane at right angles to said plane of symmetry and containing thefluke central one or a fluke longitudinal line as hereinbefore defined;and support means about which the anchor may tilt when inverted on afirm horizontal mooring bed surface to bring a lug and/or an auxiliaryfluke into contact with the mooring bed with said leading surface of theauxiliary fluke inclined upwardly at an obtuse forwardly opening angleβ₂ to the horizontal mooring bed surface.

Preferably said tilt is sustained under gravity on a firm horizontalmooring bed surface.

Preferably at least a portion of the surface of said lug facing theshank has a forward opening angle γ measured in the forward direction atright angles to the plane of symmetry in the range -2 degrees +20degrees with 4 degrees to 12 degrees preferred, and preferablyadditionally has an upwards opening angle α relative to the plane ofsymmetry in the range 0 degrees to 40 degrees measured in a planeorthogonal to the forward direction with 8 degrees to 18 degreespreferred.

Preferably the lug surface area facing the shank on each side of saidplane of symmetry is in the range of 2 to 12 per cent of the area of thefluke member viewed at right angles to the forward direction parallel tothe plane of symmetry and preferably is in the range 3 to 7 per cent ofsaid area of the fluke member.

Preferably the said forward opening angle α₁ of the leading surface ofthe auxiliary fluke is in the range 20 degrees to 80 degrees with 55degrees to 70 degrees preferred.

Preferably further the leading surface of said auxiliary fluke has adownwardly and forwardly opening angle Δ relative to the plane ofsymmetry, measured in a plane at right angles to both said leadingsurface and the plane of symmetry, in the range 60 degrees to 90 degreeswith 65 degrees to 75 degrees preferred, and preferably the flukeleading surface area at each side of said plane of symmetry does notexceed 12 per cent of the area of the fluke member viewed at rightangles to the forward direction parallel to the plane of symmetry andpreferably is in the range 2 to 7 per cent of said area of the flukemember.

Preferably said auxiliary fluke is spaced above the fluke central line.

In a preferred embodiment the support means is located such that whenthe anchor is inverted and tilted onto said lug or auxiliary fluke on afirm planar horizontal surface, the centre of area of the fluke memberis not less than 40 per cent of the fluke member width above said planarsurface and preferably not less that 50 per cent of the fluke memberwidth above said planar surface, and preferably the support means islocated such that, when the anchor is inverted on a firm horizontalplanar surface of a mooring bed and tilted thereon with said lug andsaid auxiliary fluke just buried in the bed, the portion of the flukesurface facing the mooring bed and adjacent the lug is inclined to thehorizontal at not less than 45 degrees.

Preferably the support means comprises a plate member spaced from thefluke on the plane of symmetry and extending at right angles thereto,and preferably the support means is attached to the shank.

In an alternative arrangement the support means comprises one leg of acranked shank. Preferably the fluke member has a bent up side portion towhich said lug is attached, and preferably said side portion of thefluke member forms an angle in the range 45 degrees to 90 degrees tosaid plane of symmetry.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 shows a plan view of a marine anchor according to the presentinvention;

FIG. 2 shows a side elevation of the anchor of FIG. 1;

FIG. 3 shows a front view of the anchor of FIG. 1;

FIG. 4 shows an isometric view of the anchor;

FIG. 5 shows the section A--A in FIG. 2;

FIGS. 6 to 13 show various stages by the anchor of FIG. 1 orientatingfrom an inverted position on a mooring bed surface to a buried positionin the mooring bed, FIGS. 6 to 8 being front views while FIGS. 9 to 13are isometric views;

FIGS. 14 and 15 are a side view and a partial front view respectively ofan anchor according to another embodiment;

FIG. 16 shows a side elevation of an anchor according to anotherembodiment of the present invention;

FIG. 17 shows a front view of the anchor of FIG. 16;

FIG. 18 shows a front view of an anchor according to yet a furtherembodiment of the present invention.

Referring to FIGS. 1 to 5, a marine anchor 1 for use in mooring afloating structure such as a floating oil rig, comprises a fluke member2 and a cranked shank 3 which is in accordance with the Applicant'sEuropean Patent 0020152 and which includes a longer forwardly extendingleg 4 carrying an attachment hole 5 for the anchor line (not shown) anda shorter leg 6 attached to the fluke member 2 by a bolted connection 7at upstanding flanges 8 on the fluke member 2. The fluke member 2extends transversely to a vertical plane of symmetry S--S of the anchor.

The shank 3 is of double-leg configuration formed by longitudinal plates9,10 and differently inclined strengthening plates 11, 12, 13 extendtransversely between the longitudinal plates 9, 10 and providerearwardly directed diverging passages for unobstructed soil flowthrough the shank 3. The anchor has a fluke angle θ defined by the anglebetween a line B--B extending from the cable attachment hole 5 to therear of the fluke member and the fluke central line C--C (ashereinbefore defined). Since in the present embodiment, the relevantcentral upper part of the fluke member 2 is flat this intercept will bea straight line. Also other longitudinal fluke intercept lines L--Lobtained on the fluke member by vertical planes parallel to the symmetryplane S--S are shown in FIG. 1. In this embodiment the angle θ isapproximately 28 degrees.

The fluke member 2 includes a fluke central part 14 of concave form,having a central sole 15 with upwardly bent side portions 16-17 and thesole 15 extends forwardly to two spaced toes 18, 19. The side portions16, 17 form an angle δ with the symmetry plane S--S (or plane parallelthereto) preferably lying in the range 45 degrees to 90 degrees. Thefluke central part 14 is fabricated from steel plate and includesreinforcing ribs 20 extending to the toes 18, 19. As can be seen, theupper surface of the sole 15 is flat. Additionally the fluke member 2provides upstanding lugs 21, 22 carried by respective fluke portions 16,17. Each lug 21, 22 has a forwardly opening angle δ measured in theforward direction relative to the plan of symmetry in the range -2degrees to +20 degrees and preferably 4 degrees to 12 degrees, and anupwardly opening angle α relative to the plane of symmetry in the range0 degrees to 40 degrees with 8 degrees to 18 degrees preferred. Also,the surface area of each lug 21, 22 facing the shank is in the range 2per cent to 12 per cent of the area of the fluke member viewed at rightangles to the forward direction parallel to the plane of symmetry,preferably this area is in the range 3 per cent to 7 per cent of saidfluke member area.

In accordance with the present invention, each lug 21, 22 has attachedthereto a respective auxiliary fluke (or trigger element) 23, 24 each ofwhich has a leading surface 25 which is inclined upwardly at an acuteangle forwardly opening angle β₁ to the fluke central line C--C (or afluke longitudinal line L--L). This acute angle β₁ can lie in the range20 degrees to 80 degrees, and preferably {degrees to 70 degrees. Furtherthe area of each surface 25 preferably lies in the range 1 per cent to 7per cent of the area of the fluke member 2 viewed at right angles to theforward direction parallel to the plane of symmetry (this range will beparticularly suitable for clay soils); however for soft mud mooring bedsa range up to 12 per cent may be preferable. Further, when the anchor 1lies in an inverted position as shown in FIG. 6 with an outer edge ofthe shank longer leg 4 serving to support the anchor on a firm mooringbed M (e.g. clay) but with the anchor tilted so that a lug 21 and anauxiliary fluke 23 engage the bed M, the leading surface 25 forms anobtuse angle β₂ with the mooring bed M as shown in FIG. 2. The upperends of the plates 11, 12, 13 will also assist in supporting theinverted anchor and in particular will mitigate against sinking of theshank when lying inverted on relatively soft mooring beds. Additionallythe leading surface 25 forms an angle Δ (see FIG. 5) relative to theplane of symmetry S--S measured in a plane at right angles to both saidleading surface 25 and the symmetry plane S--S: this angle Δ can lie inthe range 60 degrees to 90 degrees with 65 degrees to 75 degreespreferred.

As can be seen in FIG. 2, each auxiliary fluke 23, 24 lies above thelevel of the fluke sole portion 15 and more particularly above the levelof the fluke central line C--C. More especially each auxiliary fluke ispositioned such that it lies above the line I--I (FIG. 2) extending fromthe front end of the bottom edge of the shank longer leg 4 to the pointP on the fluke central line C--C spaced 0.66 L from the front end of thefluke member 2, where L is the length of the fluke member, and indeedthe flukes 23, 24 lie substantially above this line even when the lineextends to the rear of the fluke. The effect of this is that when theanchor 1 is burying normally, the major portion of the fluke member 2will be buried before the negatively acting auxiliary flukes 23, 24engage the mooring bed, and this combined with the fact that the area ofleading surfaces 25 is much smaller than the positive "burying" area ofthe fluke member facilitates deep burial of the anchor. The lugs 21, 22and the flukes 23, 24 are also fabricated from steel plate.

A significant feature of the above described anchor is that the leadingedges of the fluke 2 on each side of the plane of symmetry are ofindented form, that is to say the leading edges are cut back at I inplan view with respect to a straight line drawn from a front point 19(or 18) of the fluke to a point B at the outer edge of the fluke 2, i.e.at the joining edge of the plates 16, 17 and lugs 21, 22 as shown inFIG. 1. The provision of adequate such indentation of the fluke greatlyassists the rolling action of the anchor to the vertical working burialattitude.

OPERATION

FIGS. 6 to 13 show anchor orientation in firm clay. The anchor 1 isunstable when dragged inverted and quickly topples about the longer leg4 of the shank 3 until one lug (e.g. 22) and an auxiliary fluke 24 bearson the mooring bed surface M and is held there by the moment of theweight W of the anchor 1 acting about one edge of the longer leg 4 ofthe shank as shown in FIG. 6. Further dragging causes the lug 22 andauxiliary fluke 24 to penetrate the mooring bed surface M progressivelyas shown in FIGS. 7, 8 and 9 with the anchor rotating substantiallyabout the top edge of the long leg 4 of the shank in contact with themooring bed. When an appreciable portion of the fluke member 2 has beenpulled into the soil by the lug 22 and auxiliary fluke 24 as the anchoris pulled over the mooring bed surface, forces F₁, F₂ are generatedproducing a turning moment, or couple, as shown in FIG. 10. This coupleacts to lift the shank 3 off the surface of the mooring bed M as theanchor rolls further (FIG. 11) until, in FIG. 12 enough of the fluke 2is buried to pull the shorter leg 6 of the shank 3 into the soil. InFIG. 13, the rolling force F₁ from the buried auxiliary fluke 24 iscounteracted by an opposing force F₃ from the other auxiliary fluke 23with the result that the anchor 1 adopts an upright burial attitude(FIG. 13). This roll mechanism prevents the mooring bed surface reachingthe position shown dashed in FIG. 8 whereby a side face of the shank 3bears against the surface of the mooring bed and impedes rolling.Finally, the anchor proceeds to bury deeper into the mooring bed soil inthe attitude shown in FIG. 13 when pulled further by the anchor cablesince the upwards forces from the auxiliary flukes are much less thanthe downwards force produced by the main fluke due to the fact that the"negative" burial area of the auxiliary flukes 23, 24 is very much lessthan the "positive" burial area of the fluke member 2.

It is preferred in FIG. 1 that the height H of the centre of area of thefluke member 2 above the mooring bed M is not less than 40 per cent ofthe width K of the fluke member 2, and preferably not less 50 per centof K. Also as seen in FIG. 7, when the auxiliary fluke 24 and lug 22 arejust buried it is preferable that the fluke portion 17 is inclined tothe horizontal at an angle E not less than about 45 degrees. When theanchor is in the position shown in FIG. 8, it will be noted that thefluke side portion 17 is substantially vertical and in a minimumresistant position to penetration so encouraging rolling to thepositions in FIGS. 9 and 10. These features encourage the abovedescribed rolling mechanism. Additionally the flukes 23, 24 willfunction in combination with the lugs 21, 22 and the upwardly bent flukeportions 16, 17 to produce dynamic stability in the anchor 1.

The flukes 23, 24 are shown at the rear of the outer surface of the lugs21, 22 and this arrangement could provide benefits with regard to soildeflection. However the flukes 23, 24 could very well be located atother positions on the lugs, and it is felt that a substantialmid-position would be advantageous.

FIGS. 14 and 15 show the present invention applied in an anchor asdescribed in UK Patents 1356259 and 1513453. Thus the anchor of FIGS.14, 15 comprise a fluke 2A carried by single-leg cranked shank 3A withupward side lugs 30, 40 extending from the sides of the fluke 2A. Theside lugs 30, 40 carry auxiliary flukes 23A, 24A generally correspondingto the lugs 23, 24 of the FIGS. 1 to 5 embodiment, and the flukes 23A,24A will operate in a similar manner. UK Patent 1356259 teaches that theouter ends of the lugs 30, 40 can be outwardly twisted to form a greaterconvergence angle, but with the flukes 23A, 24A present this twistingcan be removed or reduced.

Modifications are of course possible. Thus, some other means than theupper edges of the shank longer leg 4 could be employed to constitutethe support means for the inverted anchor. Plate means could be used forthis purpose, and if for example a shank of straight leg form is usedthese separate plate means could be attached to the straight shank. Thefluke member 1 need not have a concave central part 15 but this partcould be fully flat with the lugs 21, 22 attached directly to this flatconstruction. Again, the fluke member 2 may be constituted purely byupstanding lug parts with no central sole.

FIGS. 16 to 19 show two anchors which are generally similar to theanchor shown in FIGS. 1 to 5 and features in FIGS. 16 to 19corresponding to the features of this previous anchor carry the samereference numerals or letters.

However, the anchor of FIGS. 16, 17 has auxiliary flukes (or triggerelements) each comprised by an outwardly extending portion 23A,(24A),equivalent to the flukes 23, 24 of the previous anchor, and additionallyby an inwardly extending portion 23B, 24B; while the anchor of FIG. 18has soley inwardly extending auxiliary flukes (or trigger elements) 23¹,24¹.

In FIGS. 16, 17 the fluke portions 23A/B, 24A/B are again carried byupstanding side lugs 21, 22 on the anchor fluke 2, and these lugs 21, 22can be set at an angle α from the vertical exactly as in the previousanchor of No. 8522062. Also, the fluke portions 23A/B, 24A/B are set atan acute angle β₁ to the fluke 2 within the range specified previouslyfor β₁ the inner and outer portions are preferably set at the same acuteangle β₁, but it would be possible to have these portions set atdifferent acute angles relative to the fluke. The forwardly facing areaof the outer portions 23A, 24A can have any suitable proportion relativeto the equivalent area of the inner portions 23B, 24B, and in theexample of FIGS. 16, 17 the outer portions have a greater forwardlyfacing area than the inner portions. With regard to the anchor of FIGS.1 to 5 the outer portions 23A, 24A of the present anchor could haveproportionally smaller area than the equivalent flukes 23, 24 of theprevious anchor. A twin-leg shank 3 of cranked form is again used inaccordance with European Patent No. 0020152.

The flukes 23A/B, 24A/B function as trigger elements when the anchorlies inverted on the sea bed and supported by an outer surface (edge) ofthe shank 3 and serve to orientate the anchor towards an upright workingburial attitude. More specifically the orientating action is asdescribed for the anchor of FIGS. 1 to 5. By providing the auxiliaryflukes 23, 24 with outer and inner portions it should be possible toreduce the stress loading on the anchor structure during anchoroperation.

Additionally the shank 3 carries laterally and rearwardly extendingsupport ears 45, 46 for an anchor chaser or retrieval device 47 (showndashed) which can be pulled down the anchor line (not shown) by means ofa pennant line 48 and onto the shank 3 to the position shown. The chaser47 is preferably constructed in accordance with UK Patent 1578129 (U.S.Pat. No. 4,098,216). The ears 45, 46 may also serve to assist anchororientation particularly when the anchor line (not shown) connected athole 5 is incident at a fairly substantial angle from the horizontal andaway from the sea bed.

In the embodiment shown in FIG. 18, the auxiliary flukes 23¹, 24¹ extendwholly inwardly from the side lugs 21, 22 towards the anchors plane ofsymmetry S--S. In this case the lugs 21, 22 are preferably set at agreater acute angle α to the vertical than previously and this will tendto ensure that the moment arm M from the centre of area of the auxiliaryfluke to the anchors plane of symmetry S--S will be substantially thesame as in the previous embodiment. The angle B₁ will be similar as inprevious embodiments. It is felt that the absence of outwardly extendingauxiliary fluke portions could convenience the handling of the anchor insome stuations.

The flukes 23¹, 24¹ will again serve as trigger elements in the mannerset out for the previous embodiment to orientate the anchor to anupright working burial attitude.

I claim:
 1. A marine anchor comprising a fluke arranged transversely toa longitudinal plane of symmetry of the anchor, a shank adapted at oneend for attachment to an anchor line and at the other end for attachmentto said fluke, said fluke including a main fluke member and fluke sidestructures on each side of said plane of symmetry, each fluke sidestructure including an upstanding lug and an auxiliary fluke attached tothe lug so as to be remote from the plane of symmetry, each auxiliaryfluke extending substantially transversely to said plane of symmetry andproviding a leading surface inclined upwardly at an acute forwardlyopening angle β₁ to a plane at right angles to said plane of symmetryand having a fluke central line; and support means about which theanchor tilts when inverted on a firm horizontal mooring be surface tobring one of said fluke side structures into contact with the mooringbed with the leading surface of the auxiliary fluke of said one flukeside structure inclined upwardly at an obtuse forwardly opening angle β₂to the horizontal mooring bed surface whereby the auxiliary fluke digsnto the mooring bed surface on forward movement of the inverted anchor.2. An anchor as claimed in claim 1, wherein said tilt is sustained undergravity on a firm horizontal mooring bed surface.
 3. An anchor asclaimed in claim 1 wherein at least a portion of the surface of eachupstanding lug of the fluke side structure has a forward opening angle γmeasured in the forward directions at right angles to the plane ofsymmetry in the range -2 degrees to +2 degrees.
 4. An anchor as claimedin claim 3, wherein said forward opening angle γ lies in the range 4degrees to 12 degrees.
 5. An anchor as claimed in claim 1 wherein atleast a portion of each upstanding lug has an upwards opening angle αrelative to said plane of symmetry in the range of 0 degrees to 40degrees measured in a plane orthogonal to the forward direction.
 6. Ananchor as claimed in claim 5, wherein said angle α lies in the range 8degrees to 18 degrees.
 7. An anchor as claimed in claim 1, wherein thelug surface area facing the shank on each side of said plane of symmetryis in the range of 2% to 12% of the area of the main fluke member viewedat right angles to the forward direction parallel to the plane ofsymmetry.
 8. An anchor as claimed in claim 1, wherein said forwardopening angle β₁ of the leading surface of the auxiliary fluke is in therange 20 degrees to 80 degrees.
 9. An anchor as claimed in claim 8,wherein said angle β₁ is in the range 55 degrees to 70 degrees.
 10. Ananchor as claimed in claim 1 wherein the leading surface of eachauxiliary fluke has a downwardly and forwardly opening angle Δ relativeto the plane of symmetry, measured in a plane at right angles to bothsaid leading surface and the plane of symmetry, in the range 60 degreesto 90 degrees.
 11. An anchor as claimed in claim 1, wherein the area ofthe leading surface of each auxiliary fluke does not exceed 12% of thearea of the main fluke member viewed at right angles to the forwarddirection parallel to the plane of symmetry.
 12. An anchor as claimed inclaim 11, wherein said leading surface area of each auxiliary fluke liesin the range 1% to 7% of the area of the main fluke member.
 13. Ananchor as claimed in claim 1, characterized in that each auxiliary flukeis spaced above the fluke central line.
 14. An anchor as claimed inclaim 1, wherein the support means are located such that when the anchoris inverted and tilted onto said fluke side structure on a firm planarhorizontal surface, the center of area of the main fluke member is notless than 40% of the main fluke member width above said planar surface.15. An anchor as claimed in claim 1, wherein the support means islocated such that, when the anchor is inverted on a firm horizontalplanar surface of a mooring bed and tilted thereon with a lug andrespective auxiliary fluke just buried in the bed, the portion of thefluke surface facing the mooring bed and adjacent the lug is included inthe horizontal at not less than 45 degrees.
 16. An anchor as claimed inclaim 1, including an upstanding support member providing said supportmeans such that said support means is spaced from the fluke.
 17. Ananchor as claimed in claim 1, wherein the support member is attached tothe shank.
 18. An anchor as claimed in claim 1, wherein the supportmeans comprises one leg of a cranked shank.
 19. An anchor as claimed inclaim 1, wherein the main fluke member has a bent up side portion towhich a lug of a fluke side structure is attached.
 20. An anchor asclaimed in claim 19, wherein said bent up side portion of the main flukemember forms an angle in the range 45 degrees to 90 degrees with saidplane of symmetry.
 21. An anchor as claimed in claim 1 wherein theauxiliary fluke has at least a portion extending outwardly from therespective lug.
 22. An anchor as claimed in claim 1, the auxiliary flukehas at least a portion extending inwardly from the respective lug. 23.An anchor as claimed in claim 1 wherein the leading edges of the mainfluke member on each side of the plane of symmetry are of indented formas viewed in plan.